Heretofore, the dichotomy has existed between urethane foams, which exhibit excellent mechanical properties (e.g. compressive strength) but are extremely flammable and produce smoke, and phenolic "resole" foams, which exhibit low combustibility and smoke but have low compressive strength and are extremely friable. Accordingly, it is desirable to provide phenolic foam having improved properties such as friability resistance and compressive strength without adversely affecting the otherwise low combustibility of phenolic foam.
Modification of phenolic foams by the incorporation of isocyanates is known in the art. For example, U.S. Pat. No. 3,872,034 discloses a phenolic foam produced by the combined reaction of a resole resin, an alkylene glycol, a phosphate flame retardant, an organotin catalyst, a silicone surfactant, an isocyanate, and a blowing agent.
U.S. Pat. No. 2,608,536 discloses the preparation of foams by heating at 150.degree.-250.degree. C. a mixture of resole resin, diisocyanate, surfactant, and nitrogenous blowing agent. The requirement that the mixture be heated to 150.degree.-250.degree. C. is inherently expensive and requires the use of special equipment for high temperature heating. Further, U.S. Pat. No. 3,298,973 discloses flame-resistant, nonpunking phenolic foams produced by oven-curing the resole in the presence of acids, an organic blowing agent, and a surfactant. The use of polyisocyanates in the production of phenolic foams is also disclosed in that patent. However, that patent uses heat curing which, as noted, is inherently expensive.
Other prior art relating to the use of an isocyanate component in the formation of phenolic foams includes:
(1) U.S. Pat. No. 2,806,006 which discloses the preparation of foam by reacting a resole resin comprising 78-85 wt. percent of the mixture with 0.5 to 10 wt. percent of an isocyanate-terminated prepolymer under heat in the presence of a blowing agent and an acid catalyst.
(2) British Pat. No. 908,303 which discloses the production of high density phenolic foams by reacting a resole resin with an isocyanate in the presence of an acid catalyst and water.
(3) U.S. Pat. No. 3,632,531 which relates to the preparation of foams by the reaction of an aromatic isocyanate with a resole resin to form a flame-retardant, heat-resistant foam. However, compressive strength is not mentioned in the patent.
None of the above-mentioned patents, however, relate to the use of phosphorus-containing isocyanate-terminated prepolymers which are employed in accordance with the teachings of this invention to provide phenolic foam of low combustibility having a good combination of friability resistance and compressive strength.